The present invention generally relates to electric induction melting, heating and stirring of an electrically conductive material, and in particular to an induction furnace with a bottom induction coil.
A material with a relatively low value of thermal conductivity, such as aluminum, can be melted and heated in a fossil fuel-fired reverberatory furnace. The salient features of a fossil fuel-fired reverberatory furnace 100 are illustrated in FIG. 1. Crucible 110 is configured to accommodate a shallow depth of molten bath 120 of the material. Heat generated by fossil fuel-fired burners 115 disposed above the surface of the bath reverberates in the volume bounded by crucible lid 125, the surface of the bath, and the side wall of crucible 110. The heat is transferred by conduction throughout the melt, with the shallow depth of the bath minimizing heat transfer time. To facilitate heat transfer from the upper to the lower regions of the bath, a mechanical stirrer 130 (shown diagrammatically in FIG. 1) is used to circulate the bath If the molten bath is aluminum, the entire bath must be kept at least above the melting point of aluminum, which is nominally 661xc2x0 C. Material charge can be added to the crucible by removing lid 125 and placing the charge in the crucible. Molten material can be tapped from the crucible at selectively closeable outlet 162.
Melting and heating aluminum in a reverberatory furnace is an inefficient process in terms of energy input, time and simplicity of operation. Additionally, mechanical stirrers are high maintenance and high failure items due to submersed operation in the molten bath. The present invention addresses these problems by providing an apparatus for and method of melting, heating and/or stirring aluminum in an efficient manner by magnetic field induction heating. The apparatus and method are also of particular value for the melting, heating and/or stirring of other metals besides aluminum and its alloys, and other electrically conductive materials having a relatively low value of thermal conductivity.
In one aspect, the present invention is apparatus for and method of melting, heating and/or stirring an electrically conductive material in an induction furnace having a bottom induction coil. The coil is placed between a bottom support structure and a magnetic flux concentrator so that a magnetic field generated external to the coil, by a current flowing through it, is directed towards the material in the crucible of the furnace to magnetically couple with it and inductively heat the material. The coil may consist of multiple active and passive coil sections. An active coil section is impedance matched to the input of an ac power supply, and the passive coil section forms an inductive/capacitive resonant circuit. Magnetic coupling of the passive coil section with a magnetic field generated by current in the active coil generates a secondary magnetic field. The fields generated by the active coil section and the passive coil section are directed towards the material in the crucible of the furnace to inductively heat the material. These and other aspects of the invention will be apparent from the following description and the appended claims.